(a) Field of the Invention
The present invention relates to a plate for use in lithographic printing made by electrophotography, in particular it relates to a plate for use in lithographic printing which comprises a photoconductive layer and a layer having a specific property (a layer whose surface changes from a hydrophobic one into a hydrophilic one) superimposed on said photoconductive layer. The term "plate" means a printing plate before plate making.
(b) Description of the Prior Art
Various type of plates for use in lithographic printing (which are sometimes called "plate" for short hereinafter) have hitherto been proposed and put to practical use. For instance,
(i) The plate comprising a substrate and a photosensitive resin layer superposed thereon, PA0 (ii) The plate comprising a substrate and a layer, superposed thereon, which is made of a silver halide photosensitive material, PA0 (iii) The plate comprising an electrically conductive substrate and a layer, superposed thereon, which is made of a photoconductive material, PA0 (1) The plate is prepared by forming a photosensitive layer (photoconductive layer) comprising zinc oxide and a binder resin on a conductive-treated substrate (electrically conductive substrate). This plate is generally prepared by forming a toner image on the photosensitive layer through the steps of electrification-imagewise exposure-development using a toner-fixing substance and successively treating the toner-free non-image area of said image so that it becomes hydrophilic by using a so-called etching liquid (for instance, an aqueous acidic solution containing ferrocyanic salt or ferricyanic salt). This printing plate, whose surface alone is subjected to hydrophilicable treatment, is profitable in that the plate making can be effected in a short length of time by means of simple equipment, but the zinc oxide-resin system photosensitive layer is insufficient in strength as the photosensitive layer is destroyed through being peeled off by mechanical pressure applied thereon during printing, by permeation of a moistening agent into the photosensitive layer and the electrically conductive substrate and attachment of ink to the non-image area takes place to thereby limit the printing durability to about 5,000-10,000 sheets. PA0 (2) The plate prepared by forming a photosensitive layer on a finely uneven aluminum plate, said photosensitive layer comprising binding an oxadiazole compound or oxazole compound with an alkali soluble resin such as a styrene-maleic anhydride copolymer or the like. This plate is made by forming a toner image on the photosensitive layer by means of electrophotography process and thereafter dissolving the photosensitive layer corresponding to a non-image area with an alkalline organic solvent and removing. However, this plate making process involves unsettled problems such as cost, safety, environmental pollution, working conditions, sanitary conditions and the like. This plate making process requires large-scale equipment due to the necessity of dissolving and removing the non-image area of the photosensitive layer, the process is time consuming and thereby slows the plate making process, and further employs ethylene glycol, glycerine, methanol, ethanol and the like as the treating liquid (said organic solvent). PA0 (3) The plate prepared by forming a photosensitive layer on a finely uneven aluminum plate, said photosensitive layer comprising a phthalocyanine pigment and an alkali-soluble phenol resin, or said photosensitive layer further containing an electron-acceptor or an electron donor. This plate making process must dissolve and remove the photosensitive layer corresponding to the non-image area. Therefore, this process has drawbacks in that it requires of a large-scale equipment and the plate making speed is slow. PA0 (4) The plate prepared by forming a photosensitive layer on the electrically conductive substrate, said photosensitive layer comprising a charge carrier generating material such as a barbituric acid derivative, a thiobarbituric acid derivative or the like and a binder resin. This plate making process has deficiencies in that it is complicated and requires a long period of time. PA0 (5) The plate prepared by forming a specific resin layer on the generally known electrophotographic element. The "specific resin layer" referred to herein is a surface layer which comprises a vinyl ether-maleic anhydride copolymer and a hydrophobic resin compatible therewith. This layer is designed so that a toner image is formed, thereafter the non-image area is treated with an alkali for hydrolytically opening the ring of the acid anhydride, and thus this layer is made hydrophilic (a hydrophilicable layer). In this plate, its printing durability was 500-600 sheets at the most. The reason for the low durability is that the vinyl ether-maleic anhydride copolymer used therein, when being in the ring-opened and hydrophilic state, becomes water soluble and even if the layer is the product of mutual solubility with another hydrophobic resin its water resisting property is markedly inferior.
and the like.
The plate (PS plate) disclosed in the preceding example (i) is superior in printing durability but inferior in sensitivity. In view of this, this plate must be exposed to a large output amount of ultraviolet rays for a long period of time at the plate making time. Further, direct plate making, wherein a plate is made directly from the original, is not applicable to this plate, a negative or a positive must be prepared once from the original by using a silver salt film. Therefore, this plate possesses a drawback in that large-scale equipment is required for making this plate, a large amount of time is consumed in making the plate.
The plate disclosed in the preceding example (ii) is made by using the diffusion-transfer developing process or the tanning development process. This plate profitably can be directly made, but has the drawbacks that printing durability is low and the cost per one piece of plate is high.
The plate disclosed in the preceding example (iii) is universally employed for the purpose of direct plate making, and various types of plates have been proposed up to now. Such plates include the inorganic photoconductive material-resin dispersion system once and the organic photoconductive compound-resin system one.
When increasing the percentage of said hydrophobic resin in the layer in order to improve the water resisting property at this stage, naturally, the hydrophilic property of the layer surface deteriorates, whereby ink tends to easily adhere to the non-image area and the printed matter is apt to become stained. Therefore, it is difficult to meet both the hydrophilic and the water resisting property requisites for the lithographic printing plate by blending the hydrophilic but water resisting property-free resin with the water-resisting but hydrophobic resin. This procedure can never solve this fundamental problem.